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To clarify the photosynthetic physiological response of mulberry seedlings under mixed saline–alkali stress, different saline–alkali combinations were prepared to study its effect on leaf water content, chlorophyll content, and changes of chlorophyll fluorescence parameters of mulberry seedlings leaves. Different saline–alkali combinations were composed of three salt concentrations (50, 100, and 200 mmol L−1) and four pH gradients (7.0, 8.0, 9.0, and 10.0) in different proportions, of which salt concentrations were prepared with two neutral salts (NaCl and Na2SO4) and two alkaline salts (Na2CO3 and NaHCO3) in different proportions. The results showed that different pH levels had relatively limited influence on chlorophyll fluorescence parameters of mulberry seedling leaves when the salt concentration was 50 mmol L−1 and 100 mmol L−1, but the photochemical activity decreased slightly with the increase of the pH value in the 100 mmol L−1 treatment. When salt concentrations increased to 200 mmol L−1, the photochemical efficiency of PSII in mulberry seedling leaves decreased significantly. With the increase of the pH value, the reduction of the PSII reaction center activity in mulberry seedling leaves was accelerated. Mixed saline–alkali stress also resulted in a significant decline in the electron transport rate (ETR) of mulberry seedling leaves, which was mainly due to the decrease in activity of the oxygen-evolving complex (OEC) on the PSII donor side and the blockage of electron transfer from QA to QB on the PSII acceptor side. This electron transfer on the PSII acceptor side was the main target point of saline–alkali stress. At a salt concentration of 100 mmol L−1, mulberry seedlings dissipated excess light energy by increasing non-photochemical quenching (NPQ). However, at the 200 mmol L−1 salt concentration, the protective ability of NPQ decreased, and the reduction was more significant at a high pH value, which led to the accumulation of excess excitation energy (1 − qP)/NPQ and the aggravation of photoinhibition. Saline–alkali mixed stress also significantly changed the energy distribution parameters of the PSII reaction center. A higher salt concentration and pH value led to inactivation or degradation of the PSII reaction center and reduced the ability of antenna pigment to capture light energy. Under the interaction of high salt concentration and high pH value, mulberry seedlings dissipated excess excitation energy mainly in the form of heat energy with the decrease of NPQ. In conclusion, the effects of salt and pH value on the PSII function in mulberry seedlings leaves were minimal under low salt concentrations, and there was no significant interaction between them. However, salt and pH showed significant interactions under the high salt concentration, and the higher pH value affected photoinhibition in PSII under saline–alkali stress. Therefore, the influence of total alkalinity and alkalinity should be considered when planting mulberry in high salinity areas.

The effects of arbuscular mycorrhizal fungi Glomus mosseae (+AMF) on the growth, nutrient uptake and leaf photosynthetic characteristics of alfalfa (Medicago sativa) under salt stress were studied. In alfalfa, 100 mM salt stress had no obvious effect on plant growth, nutrient content and photosynthetic capacity, however, 200 mM salt stress significantly inhibited biomass accumulation and NPK uptake. In addition, 200 mM salt stress led to decreased photosynthetic carbon assimilation capacity and lower PSII and PSI activity levels. Cultivation of alfafa with +AMF promoted biomass accumulation and uptake of nutrients, increased its underground growth, and promoted accumulation of P over N and K. +AMF increased PSII and PSI activities in alfalfa leaves under salt stress, and increased the electron transfer ability of PSII donor and acceptor sides. +AMF not only made alfalfa have relatively high stomatal conductance under salt stress, but also significantly enhanced its ability to utilize CO2.

In this paper, the effects of 100 mM NaCl and NaHCO3 stress on reactive oxygen species (ROS) and physiological and proteomic aspects of ROS metabolism in mulberry seedling leaves were studied. The results showed that NaCl stress had little effect on photosynthesis and respiration of mulberry seedling leaves. Superoxide dismutase (SOD) activity and the expression of related proteins in leaves increased by varying degrees, and accumulation of superoxide anion (O2·-) not observed. Under NaHCO3 stress, photosynthesis and respiration were significantly inhibited, while the rate of O2·- production rate and H2O2 content increased. The activity of catalase (CAT) and the expression of CAT (W9RJ43) increased under NaCl stress. In response to NaHCO3 stress, the activity and expression of CAT were significantly decreased, but the ability of H2O2 scavenging of peroxidase (POD) was enhanced. The ascorbic acid-glutathione (AsA-GSH) cycle in mulberry seedling leaves was enhancement in both NaCl and NaHCO3 stress. The expression of 2-Cys peroxiredoxin BAS1 (2-Cys Prx BAS1), together with thioredoxin F (TrxF), thioredoxin O1 (TrxO1), thioredoxin-like protein CITRX (Trx CITRX), and thioredoxin-like protein CDSP32 (Trx CDSP32) were significantly increased under NaCl stress. Under NaHCO3 stress, the expression of the electron donor of ferredoxin-thioredoxin reductase (FTR), together with Trx-related proteins, such as thioredoxin M (TrxM), thioredoxin M4 (TrxM4), thioredoxin X (TrxX), TrxF, and Trx CSDP32 were significantly decreased, suggesting that the thioredoxin-peroxiredoxin (Trx-Prx) pathway's function of scavenging H2O2 of in mulberry seedling leaves was inhibited. Taken together, under NaCl stress, excessive production of O2·- mulberry seedlings leaves was inhibited, and H2O2 was effectively scavenged by CAT, AsA-GSH cycle and Trx-Prx pathway. Under NaHCO3 stress, despite the enhanced functions of POD and AsA-GSH cycle, the scavenging of O2·- by SOD was not effective, and that of H2O2 by CAT and Trx-Prx pathway were inhibited; and in turn, the oxidative damage to mulberry seedling leaves could not be reduced.

In order to explore the response and adaptation mechanisms of photosynthesis of the leaves of mulberry (Morus alba L.) seedlings to saline-alkali stress. Photosynthetic activity, and the response of related proteomics of M. alba seedling leaves under NaCl and NaHCO3 stress were studied by using chlorophyll fluorescence and gas exchange technique combined with TMT proteomics. The results showed that NaCl stress had no significant effect on photosystem II (PSII) activity in M. alba seedling leaves. In addition, the expressions of proteins of the PSII oxygen-evolving complex (OEE3-1 and PPD4) and the LHCII antenna (CP24 10A, CP26, and CP29) were increased, and the photosystem I (PSI) activity in the leaves of M. alba seedlings was increased, as well as expressions of proteins, such as PsaF, PsaG, PsaH, PsaL, PsaN, and Ycf4. Under NaHCO3 stress, the activity of PSII and PSI and the expression of their protein complexes and the electron transfer-related proteins significantly decreased. NaCl stress had little effect on RuBP regeneration during dark reaction in the leaves and the expressions of glucose synthesis related proteins and net photosynthetic rate (Pn) did not decrease significantly. The leaves could adapt to NaCl stress by reducing stomatal conductance (Gs) and increasing water use efficiency (WUE). Under NaHCO3 stress, the expression of dark reaction-related proteins was mostly down-regulated, while Gs was reduced, which indicated that non-stomatal factors can be responsible for inhibition of carbon assimilation.